Apparatus for testing insulation of electrical cables



2, 1948. K. H. ZIMMERMANN 2,452,624

APPARATUS FOR TESTING INSULATION OF ELECTRICAL CABLES Filed June 11, 1945 IN V EN TOR. A A 1?!- M Z IMME fiM/l/V ATTORNEY Patented Nov. 2, 1 9

APPARATUS FOR TESTING INSULATION OF ELECTRICAL CABLES Karl H. Zimmermann, Nutley, N. 3., assignor to Federal Telephone and Radio Corporation,

New York, N. Y., a corporation ofDelaware Application June 11, 1945, Serial No. 598,688

This invention relates to the testing of the dielectric strength of the insulation on cables or the like. An object of this invention is to prosuch testing with the dielectric immersed in water and at various controlled temperatures. A further object is to provide apparatus for such testing which is simple and sturdy in construction and yet is easy to use and dependable in operation.

The invention accordingly consists in the features of construction, combinations of elements, arrangements of parts and in the several steps and relation and order of each of the same to one or more of the others, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claim.

In the drawing, the single figure is a side elevation of a test unit forming on embodiment of the invention with parts broken away to show the internal construction, and with certain of the apparatus represented schematically.

The dielectric strength of insulating materials may be tested by placing it between two electrodes -pendab1e' results.

and then increasing the potential between the electrodes until there is a breakdown through the dielectric, the breakdown being evidenced by'the forming of an electric arc through, or rupture of, the insulating material. With these tests, it is often difllcult to determine the exact conditions existing at the point in the dielectric where the breakdown occurs. pointed out that the dielectric strength of many materials changes inversely with changes in temperature, and therefore, if the insulation material becomes heated at a particular point during the test, the breakdown is apt to occur there. With certain prior test systems, the test operation heats up the insulating material, and often an excessive heating at one point causes the insulating material to become much hotter there than elsewhere. Furthermore, the physical characteristics of the insulating material may vary from point-to-point and in a layer of insulating material the thickness may vary, and all such factors affect the dielectric strength under test. These factors may or may not change the test results correspondingly. The test results are apt to be inaccurate where the test conditions are such that it is difficult to determine the temperature of the dielectric material at the exact point where the breakdown occurs. In addition, it is important to note In this connection, it is, P

1 Claim. (Cl. 175-183) that with certain prior test systems, the tests are not thorough and may not disclose the weakest point in the insulating material.

When a particular insulating material is to be used only under certain specific conditions, as at a relatively high or a relatively low temperature, it is important that the test be carried on under conditions which simulate the conditions of use. That is, a series of tests at room temperature might give results having no correlation to the results which would be obtained under the conditions of actual use. In accordance with the present invention, the tests care carried on in a simple manner which avoids the difiiculties of the prior art and which gives accurate and de- In the illustrative embodiment of the present invention the dielectric strength 01' the insulation of a single conductor cable is tested. This conductor forms one electrode; another is needed, and therefore, during test, it is immersed in water. Furthermore, the cable is maintained at a predetermined temperature which is indicated by a thermometer, there being heating means to increase the temperature of the cable. The breakdown normally occurs in the warmest zone, and the arrangement is such that the warmest zone is where the temperature is measured. Furthermore, the warmest zone is below the surface of the water so that the danger of flash-over" along the surface of the water is avoided.

Referring to the drawing a U-tube I of copper tubing has two legs 2 and 3 connected at the bottom by a cross-member 4, and except for the upper ends of the legs, the tube is enclosed in a heating Jacket 5. Heating jacket 5 is formed by a pair of concentric coverings 6 and I having a heating layer 8 therebetween formed by an evenly spaced winding of high resistance wire. Positioned between the two legs of the U-tube is a thermometer well formed by a glass tube I0 opening at its lower end into U-tube I through a sealing ferrule ll. Supported at the top of tube I 0 by clip l2 and extending to the bottom of the well into the U-tube is a thermometer l3, which measures and indicates the temperature at the center of the cross-member 4 of the U-tube.

Attached to the upper ends of the two legs 2 and 3 of the U-tube are two funnel-shaped endbells i4 and tached to its leg, as shown at the left in the figure, by a flanged collar I 6 carried by the endbell and threaded onto a flange at the upper end of the leg. End-bells I 4 and i5 are provided at their tops with rings l1 and i8, welded to the i5. Each of these end-bells is atrespective end-bells. These rings provide a comparatlvely large radius of curvature and prevent flash-over from the top edges of the end-bells.

Electric current is supplied to heater winding as shown schematically at the left, through a pair of leads l9 and 20 connected through an adjustable resistor 2| and a switch 22 to a source of power. The U-tube is filled with water, and a single conductor insulated cable 23 having a wire 24 is shown positioned in the U-tube for test. The temperature of the water at the bottom of the U-tube is indicated by thermometer I3, and resistor 2| is so adjusted as to maintain the desired temperature.

The test potential is impressed between the wire 24 and end-bell H so that the wire within the cable is at one potential and the water surrounding the cable is at ground potential. The connections for this high potential are made by a, pair of clamps (not shown) and a pair of wires 25 and 28 which extend from the secondary winding 21 of a high potential transformer 28. The

primary winding 29 of the transformer is connected through an induction regulator 39 and a switch 3| to a source of alternating current.

As indicated above, with this arrangement the water maintains the insulation on cable 23 at the temperature indicated by thermometer l3 which is at the warmest zone at the bottom of the U- tube; thus, the breakdown will normally occur at the point where the temperature is known. To make a test, the operator closes switch 22 for a sumcient time to heat the water and the cable to the desired temperature. Thereafter, switch 3| is closed with the minimum voltage across the secondary winding 21. The voltage is gradually raised by adjusting the induction regulator until a, breakdown occurs at which time switch 3! is opened. The water acts as the outer electrode to impress the voltage over the entire outer surface of the cable. Furthermore, the water insures that the insulating material will be maintained at the desired temperature throughout and no hot spots" will be created. The endbells facilitate the filling of the U-tube with water, and flash-over from the conductor to the upper edges of the end-bells is prevented by rings H and I8. Ii is understood that with both.

of the current supply systems more complicated and more accurately controlled arrangements may be provided, but these arrangements are known to those skilled in the art.

As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, al without departing from the scope of the invention, it is to be understood that all matter hereinabove set forth, or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

In test apparatus of the character described. the combination of, a substantially U-shaped tube having legs adapted to be positioned upright with openings at the upper ends of its legs whereby an insulated conductor may be passed through the tube, a pair of end-bells attached to said upper ends and flaring outwardly therefrom, a heater jacket formed by an electric heater covered by heat-insulating material and substantially enclosing said tube whereby its temperature may be controlled, a tube parallel to said legs and open at its lower end to the central portion of said tube thereby to provide a thermometer-well, and means to impress a test potential between the tube and a conductor placed therein whereby the insulation on the conductor is tested.

KARL H. ZHJIVIERMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,565,613 Anderegg Dec. 15, 1925 1,721,374 Dantsizen July 18, 1929 1,809,714 Mathews June 9, 1931 1,880,917 Eastlake Oct. 4, 1932 1,944,211 Brodie Jan. 23, 1934 OTHER REFERENCES Publication titled Testing Insulation in General Electric Review for Nov. 1944. 

